The present invention belongs to the art of multiple access communications systems such as indoor wireless communications (wireless PABX's), wireless Local Area Networks (Wireless LANS), cellular landmobile communications systems and mobile satellite communications systems. Such systems are characterized by at least one fixed base or relay station attempting to maintain communications with a plurality of subscriber stations or terminals. These fixed stations are arranged to provide area coverage, for example to cover different countries in the case of a satellite communications system, to cover different cities in the case of cellular communications system networks, or to cover different floors or corridors in the case of an indoor wireless network.
In such systems, capacity to support a large number of subscribers is measured in units such as Erlangs per MHz per square kdlometer. Thus the capacity can be increased by using more MHz of bandwidth, or by reducing the area covered by each base station so that there are more base stations per square kilometer. The latter technique, namely of reducing cell size, increases the infrastructure cost, so it is a better measure of the cost-efficiency of a multiple access system to express capacity as Erlangs per MHz per cell.
If adjacent cells use the same frequency, interference between those cells can occur. On the other hand, if adjacent cells are not permitted to use the same frequency, by implementing a so-called frequency re-use plan that guarantees a minimum spatial separation of N cells between cells using the same channels, then the number of channels available in any one cell is reduced by N squared. In the parent application, it is shown that coding signals by adding redundancy and expanding their bandwidths such that they can tolerate adjacent cell interference leads to greater capacity than minimizing bandwidth at the expense of interference tolerance and being thereby forced to use frequency planning. This result relies on being able to avoid worst case interference situations such as two terminals on the border between two cells using the same frequency. Geographical proximity of terminals is referred to herein as terminals which are "cosited". The parent application avoids these situations by, for example, a sorting process that allocates terminals to frequencies such that almost-cosited terminals are not allocated the same frequency. The parent application also describes, among other things, two different ways to utilize multi-element antenna arrays to resolve co-channel interference.
In a first method, each antenna element receives a different combination of signals from a number of terminals. When the number of antenna elements is at least as great as the number of co-channel signals, this results in a set of simultaneous equations that can be solved to separate the individual terminal signals, thus undoing the process of co-channel signal mixing that takes place in the aether, providing terminals are not cosited.
In the cosited case, the matrix of signal coefficients becomes singular and cannot be inverted, and the equations cannot be solved. For that case, the parent application discloses an alternative method in which it is not attempted to undo the process of co-channel signal mixing that takes place in the aether; rather, the process of mixing is applied to hypotheses of the co-channel signals to predict what should be received at each antenna element, and a mismatch between the predictions and the actually received signals is calculated as an indication of the likelihood of each hypothesis. The hypothesis which results in the lowest cumulative mismatch over several symbol periods is then determined by applying the Sequential Maximum Likelihood Sequence Estimation algorithm, commonly called the Viterbi algorithm or the Dynamic Programming algorithm (DP).
In the parent disclosure, use of the Viterbi algorithm for jointly demodulating co-channel signals received by an antenna array was described for the case of a number of co-channel signals received at a base station multi-element antenna array. The algorithm complexity for binary signals was on the order of 2 to the power of the number of co-channel signals, thus imposing a practical difficulty on processing a large number of co-channel signals received at a base station due to the doubling of complexity for each added signal.